Liam Fox, the Tory defence spokesman, is calling for Britain to shorten its deployment in Afghanistan by setting clear targets for military success and sending more troops to train the Afghan army.

His words will be seen as moving towards a more populist emphasis on disentangling the UK from a conflict which increasingly lacks public support, as Cameron condemns the ‘scandal’ of UK helicopter shortage in Afghanistan.

In an interview with the Observer, Fox declined to set any timetable for an exit but said the allies should define their “benchmarks” for military success and thus the end of the mission, the tactic used by Tony Blair when under pressure to withdraw troops from Iraq.

But he suggested “the most important political decisions” would be taken in Washington, not London, and that Britain was not “the prime driver”, a surprisingly frank assessment.

“Clearly, if our mission is to create a stable Afghanistan state, the quicker we are able to provide them with the tools they need, the shorter our military involvement is likely to be,” Fox said. “And the slower the process of building up the Afghan national army, the longer our deployment is going to be.

“Given the state of Afghan society and its economic development, the international community is likely to be involved there for a very long time â€¦ but that’s a very different question from how long our military will be involved.

“Setting artificial timetables, particularly for political reasons, runs the risk of saying to our enemies: ‘If you can outlast us we will run.’ You set benchmarks and objectives, then you work towards them and constantly appraise your progress.”

He confirmed he had told US generals that a Conservative government would be “much more sympathetic” than Gordon Brown to requests for more soldiers, who would be deployed to speed up the training of an Afghan army that could control its own security.

However, he warned that the end of the war in Afghanistan would not mean the end of the war on Islamist terrorism. “In the cold war we weren’t dealing just with Russia, we had to deal with satellites too. We will have to do the same in this war,” he said. “We will face difficulties on a number of fronts and we will simply have to find the political and the military resilience to deal with it.”

Public support for the war should be bolstered by explaining that British soldiers were fighting to avoid a strategic defeat for Nato, shattering its credibility as a deterrent force, he added.

This month, President Barack Obama will receive a report from General Stanley McChrystal, commander on the ground in Afghanistan, that is likely to urge a rethink of military strategy and yet another “surge” of up to 10,000 troops to speed up the process of handing Afghanistan back.

Asked if the Tories would support sending more troops, Fox said it would depend on the request, but added: “I said to General McChrystal that had we been the government and had we been asked for more troops to accelerate the training, we would have been much more sympathetic than the current government, and we maintain that view.”

A ComRes poll for the Independent on Sunday last night found that 60% of Britons believed UK forces should be withdrawn as quickly as possible â€“ down four points on last month â€“ while the number who disagreed was unchanged on 33%.